Hey guys. My wife and I got married this summer and just bought our first house. Temperatures recently started to drop below 32F and the furnace is getting a bit more work. It's a Heil DC80 Nat. Gas unit, and I believe it was installed in 1998. I'm a certified Auto tech so I'm good with my hands and hoping to be at least a slightly useful handyman around the house....

The problem I have is that the furnace sometimes blows cold air. Both times it has happened has been after it was off for a while. We had the heat off Sunday b/c we weren't home, and turned the thermostat back up when we got home, and it blew heat for a while, but then cold air- the digital tstat would read "System On", but only the fan on the furnace was going.

This morning I got up to go to work and found it blowing cold air again (after it had been turned down for the night). We have a programmable thermostat set to 66 day and 61 night. I turned the tstat way down and shut power to the furnace off. Waited two minutes, turned power back on and turned the tstat up. The pilot light lit, after about 30 seconds the main burners? came on, and a minute later the fan started blowing warm air into the house. It lasted about 20 minutes and then it was blowing cold again. When I looked into the sight glass, there was no flame. I turned off power and gas, and pulled the cover with the sight glass in it (10 screws, in my first picture below). I blew it out as best as I could, started the furnace again and went to work. My wife called about 30 minutes later saying it was blowing cold again. I told her to turn the tstat down to current house temperature (61).

When I got home, I found that if I left that tstat at 61, the furnace would cycle for about 5 minutes every 20 mins (guessing on the times here), and the house would maintain a steady temperature. At my father-in-laws advice, I turned the stat up to 90 and went outside for a while to shovel snow. When I came back in, the house was at 65, and still blowing warm. I set the tstat to 65, and left for hockey- so far the house has been holding at 65 ever since.

In the first picture to the right of the "box", three wires enter (two blue, one black) that come from the gas valve above it. It doesn't seem like where they enter is sealed too well. Could this be an issue?

I did install a new filter. Actually didn't have any problems UNTIL I installed a new filter.

I've been reading a lot of posts that mention a flame sensor.. Could this be my issue?

check the obvious..... is filter clean...cooling coil clean.... looks like you have the famous smart valve there...... they can do strange things

chwkrx7

11-03-2012 01:46 PM

Yup, filter is clean. As for checking the obvious, I'm a total "noob", so I'm not 100% sure what the obvious is yet- just from searching and reading what I can find. Where would the cooling coil be? Is that brown-ish colored box the "smart valve"? Thanks

yuri

11-03-2012 02:02 PM

what type of thermostat do U have. if it is an older or el cheapo one then it may be shorting and energizing the fan (G) terminal and running the fan. check the temp rise thru the furnace, if it is overheating then it may be opening the limit control below the exhaust fan. you can check that when the fan is stuck on. should NOT have 24 volts AC across it at that time. to check the temp rise you stick a cooking thermometer in the supply duct about a foot downstream from the plenum and subtract the house, return temp and check the temp rise specs on the sticker. usually 35-65F. if overheatin then the ac coil may be dirty or vents closed. the smartvalve is prone to problems but the fan timer board for that furnace is very reliable. good luck

carmon

11-03-2012 02:18 PM

Quote:

Originally Posted by yuri
(Post 1043891)

what type of thermostat do U have. if it is an older or el cheapo one then it may be shorting and energizing the fan (G) terminal and running the fan. check the temp rise thru the furnace, if it is overheating then it may be opening the limit control below the exhaust fan. you can check that when the fan is stuck on. should NOT have 24 volts AC across it at that time. to check the temp rise you stick a cooking thermometer in the supply duct about a foot downstream from the plenum and subtract the house, return temp and check the temp rise specs on the sticker. usually 35-65F. if overheatin then the ac coil may be dirty or vents closed. the smartvalve is prone to problems but the fan timer board for that furnace is very reliable. good luck

Yuri .... welcome back.....

how

11-03-2012 02:24 PM

If you think the thermostat is the cause of the blowing cold air, tape in the furnace door switch so the next time this problem happens, you can open the door and remove the thermostat line to the "G" connection on the furnace. Do not touch any other connection but the "G" line. If the blower stops, you will know the problem is related to the thermostat or it's lines.

Yuri! where you've been? We even unfairly maligned lennox on numerous occaissions just to see if you were still out there.

yuri

11-03-2012 02:26 PM

1 Attachment(s)

Thankx. I went on a sabbatical to the old country and am now back with no allegiance to Lennox as I am an independant now.:no::thumbup:

yuri

11-03-2012 02:30 PM

I somewhat still like Lennox but put a nice 2 stage Rheem 95% unit in my sisters house a few weeks ago. Good price and value for the $$. I have been watching the Highway to Hell show about your Coquhalla Hwy in the Winter and heavy resuce towing. Dangerous road!! Much safer where I am and the dog sleds don`t go so fast.:laughing:

carmon

11-03-2012 04:16 PM

YA Rheem is o.k. .... hard to get the labor warranty.... I like Lennox as well ....zero failures so far......

chwkrx7

11-03-2012 05:17 PM

Thanks Yuri, I'll have to do some digging and see what I can come up with I guess.

As for the tstat it's an older Honeywell... I found an exact picture of it: